Abstract
The thermo-mechanical control processing of Nb-Ti micro-alloyed steel by induction heating in the endless strip production (ESP) line was analyzed to better understand the microstructural evolution and Nb precipitation and dissolution behavior in austenite during rapid heating to high temperatures. The Nb-Ti micro-alloyed steel consisting of 0.05 wt% C and 0.05 wt% Nb was processed through simulated rough rolling at 1050 °C followed by rapid isothermal reheating at 1150 °C. The austenite coarsening behavior and the Nb dissolution behavior at different holding times were compared, and the coarsening kinetics of austenite grains and the dissolution kinetics of precipitates were investigated. It was found that during induction heating, the size of austenite grains gradually increased with the isothermal time, and the amounts of precipitates were greatly reduced. Round precipitates of (Ti, Nb) (C, N) and square precipitates of Ti (C, N) gradually dissolved into the austenite matrix with the holding time. The Nb content in the solution increased from 0.0137 to 0.0299 wt% as the holding time increased from 1 to 40 s; therefore, about 59.8% of the total Nb content dissolved into the austenite matrix during the induction heating process.
Highlights
Microalloying is a valid method to enhance the mechanical properties of steel
Highstrength low-alloy (HSLA) steels micro-alloyed with Nb, Ti, and V are extensively applied to automobiles, ships, oil, and gas transmission lines [1,2,3,4]
The results have shown that the fine austenite grain size in microalloyed steel at the lower soaking temperature can be attributed to the pinning effect from Nb (C, N) and V (C, N) precipitates
Summary
Microalloying is a valid method to enhance the mechanical properties of steel. Highstrength low-alloy (HSLA) steels micro-alloyed with Nb, Ti, and V are extensively applied to automobiles, ships, oil, and gas transmission lines [1,2,3,4]. The results have shown that the fine austenite grain size in microalloyed steel at the lower soaking temperature can be attributed to the pinning effect from Nb (C, N) and V (C, N) precipitates. Ti has frequently been added to HSLA steels to control the grain sizes of austenite and transformed ferrite during hot deformation, as well as the subsequent heat treatment. In the ESP line, the induction heating process between rough rolling and finish rolling can rapidly reheat the intermediate billet to the required finish rolling entry temperature, improve the productivity and reduce the energy consumption for hot strip production, and maximize the microalloying effects during subsequent finish rolling, laminar cooling, and coiling by deformation-induced ferrite transformation (DIFT) and strain-induced precipitation (SIP) [18]. The research results will provide a guidance for improving the mechanical properties of Nb-Ti microalloyed steel
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